As the ecologists Yrjö Haila and Richard Levins have pointed out, “ecology” has at least four meanings, each of which is relevant in the context of discussions of contemporary biodiversity conservation and other environmental problems:

• ecology the nature: nature's economy as a material fact, and as a material basis for human existence;

• ecology the science: the biological discipline investigating nature's economy;

• ecology the idea: prescriptive views of human existence, derived from what is known or believed about nature's economy; and

• ecology the movement: political activities trying to transform society to agree with ecological ideals.

If the first use of “ecology” is meant to imply that substantive claims can be made about the “material” facts of ecology, independent of the scientific or descriptive framework invoked in the second use of “ecology” as a science, the distinction made between these two uses involves an acceptance of at least a modest version of the philosophical doctrine of realism: the claim that it is possible to make claims about the world independent of our descriptions of it. In the interest of avoiding contentious philosophical issues that do not directly impinge on the aims of this book, no position will be taken here on the tenability of this doctrine. Rather, except when otherwise indicated, “ecology” will be used in the second sense. Moreover, because it will be assumed that biologically correct descriptions are those that withstand empirical testing against nature, whatever value the first use has will be implicitly subsumed under the second.